Final answer:
The function f(x) is differentiable at every point in its domain, which is from 0 to 20, inclusive.
Step-by-step explanation:
In mathematics, a function is not differentiable at a point if its derivative does not exist at that point. Since the graph of f(x) is a horizontal line for 0 ≤ x ≤ 20, the function f(x) is a constant function. As a constant function, the derivative of f(x) is always zero. Therefore, f(x) is differentiable at every point in its domain, which is from 0 to 20, inclusive.